Peering into the violent universe

Peering into the violent universe

Tests prove new gamma-ray telescope
design

Feb. 9, 1999: A new approach to observing the most
violent objects in the sky is taking shape and will be reviewed
with competing concepts today near Washington. Fiber GLAST -
the scintillating fiber detector concept for the next-generation
Gamma-ray Large Area Space Telescope - has passed a number of
tests that show the basic concept is good and could meet the
program's goals in space.

"We're finalizing our design to
respond to the Announcement of Opportunity that will lead to
a flight project," said Dr. Geoff Pendleton, the principal
investigator for FiberGLAST. Pendleton is a gamma-ray astrophysicist
with the University of Alabama in Huntsville. He works at NASA's
Marshall Space Flight Center.

Right: Pendleton examines optical fiber bundles emerging
from a metal stack (left) and feeding into a camera (center)
in an arrangement similar to what is planned for FiberGLAST.
GLAST would produce images similar to the background image, top,
a simulation of gamma-ray sources in the direction opposite the
center of our galaxy.

Observations by the Compton Gamma Ray Observatory and other
spacecraft over the last few years have provided some of the
answers that scientists have sought about the high-energy universe.
But they've also raised new questions. A number - including what's
at the heart of many gamma-ray sources - have eluded answers.

To help go after those answers, NASA's Goddard
Space Flight Center in Greenbelt, Md., is developing GLAST to
see finer details than Compton can resolve. It is part of NASA's
initiative to study the structure and evolution of the universe.

In 1998, three science teams, including one led by Pendleton,
were awarded initial technology development contracts to pursue
promising concepts for GLAST.

"In addition to fundamental instrument performance tests,
we've done a lot of system engineering," Pendleton said,
"which is a significant part of developing a flight instrument."

That work has been led by the
University of New Hampshire, with help from the entire team,
including NASA/Marshall, UAH, Louisiana State University, Washington
University in St Louis, the University of California at Riverside,
and NOVA R&D, Inc.

"We have a great deal of flight experience on our
project," Pendleton said. "It's a pretty good cross-section
of what's flown in high-energy astrophysics." The resume
includes missions such as the Compton Gamma Ray Observatory,
Skylab, the Solar Maximum Mission, and the High Energy Astronomy
Observatories.

Like many ideas for new science instruments, the basic idea
is simple. Building FiberGLAST and making it work will be the
challenge.

Gamma rays are impossible to focus even with advanced mirrors
like the Chandra X-ray Observatory will use. Instead, they are
observed indirectly, by measuring light flashes generated as
gamma rays hit solid matter. In FiberGLAST, the solid matter will be
layers of heavy metal plates. Each interception turns one energetic
gamma-ray photon into a series of energetic particles that produce
light flashes as they pass through layers of plastic fibers sandwiched
between the metal plates. The fibers, arranged at right angles
to each other, paint a picture of the particle shower as it expands
through the FiberGLAST detector. It's almost like watching a
sky rocket blossom into a cascade of sparkles.

Right: Optical fibers sprout from between metal plates
in a 1998 test of the FiberGLAST design.

Demonstrating that this would
work was one of the first tests for the FiberGLAST concept. In
1998, the FiberGLAST team took a small working model to the Continuous
Electron Beam Accelerator Facility at the Thomas Jefferson National
Laboratory in Newport News, Va. Because the accelerator produces
a precisely controlled stream of electrons, it can also produce
a gamma-ray beam of precise energy when the electrons hit the
right target. And that lets scientists measure, precisely, the
performance of a gamma-ray detector.

"It
worked very well," Pendleton said. "The fibers demonstrated
high efficiency for detecting the gamma-rays."

Left: Simulations predict that FiberGLAST performs
far better than the minimum "baseline" requirement
set for the GLAST mission.

But FiberGLAST is more than plastic fibers between metal plates.
The light flashes travel down the fibers to multiple anode photomultiplier
tubes, the complex, sensitive light detectors.

"We ran a vibration test to simulate launch on the Delta
II rocket that will most likely carry GLAST," Pendleton
said. "The concern was that it could fall apart. The test
proved to the scientific community that our readout system could
survive launch."

Once the light flashes reach the detectors arrays, most have
to be discarded because they'll really be noise.

Science objectives for GLAST

GLAST will identify and study
nature's high-energy particle accelerators through observations
of active galactic nuclei, pulsars, stellar-mass black holes,
supernova remnants, gamma-ray bursts, and diffuse galactic and
extragalactic high-energy radiation in the energy range from
20 MeV to 100 GeV and higher. GLAST will use these sources to
probe important physical parameters of the Galaxy and the Universe
that are not readily measured with other observations. The high-energy
gamma-rays will be used to search for a variety of fundamentally
new phenomena, such as particle dark matter and evaporating black
holes. Scientific objectives include:

How
do active galactic nuclei (AGNs), such as blazars (as depicted
in the artist's concept at right) form and evolve?

What powers the jets emanating
from AGNs and galactic black holes and how are the particles
in the jets accelerated? How are these structures connected with
similar structures seen at smaller scales?

At what energies are the breaks
in the gamma-ray spectra of AGNs? Are high energy spectral cutoffs
due to source-intrinsic absorption effects or to absorption by
extragalactic background light? What is the redshift dependence
of these effects? Is there a class of AGNs that can be used as
high-energy "standard candles"?

What is the origin of the isotropic
"diffuse" gamma-ray background?

What are the sites and mechanisms
of cosmic-ray acceleration? How do rotation-powered pulsars generate
high-energy gamma-rays and what is the relation of this radiation
to emission in lower energy bands?

What is the rate of supernovae
in the Galaxy and where are the unobserved supernovae of the
past several hundred years?

What are gamma-ray bursts and
how do they generate high-energy radiation?

What are the unidentified high-energy
gamma-ray sources in our galaxy?

"The biggest problem is cosmic rays which go through
our system 1,000 to 10,000 times as frequently as gamma rays,"
Pendleton said. It's a bit like looking for a few drops of rain
in the middle of a hail storm.

Most of the cosmic rays are
protons zipping along with energies of 4 to 100 billion electron
volts (4-100 GeV; a photon of visible light carries about 2 electron-volts
of energy).

"If they aren't rejected by the detector system,"
Pendleton said, "they'll overload the data acquisition and
processing system. That system has to be able to make a quick
veto of almost all the protons that go through the system."

Simulating everything that happens when both cosmic and gamma
rays hit GLAST has been a challenge. Each photon or particle
will generate an expanding trail of debris through the entire
apparatus. Each encounter has to be observed and quickly recorded
or discarded.

"Marshall has a 100 terabyte (100 trillion bytes) data
base storage system," Pendleton said. "There aren't
many of those around." By way of comparison, it would take
almost 667 million floppy disks to store that much data.

"We needed it to develop the hardware readout system,"
Pendleton said. "In it, we've modeled tens of millions of
events. These are large amounts of data that take months to generate
in a computer with 17 processors and 1 gigabyte of RAM."

The tests have demonstrated that the team's approach to FiberGLAST
will weed out the noise and return just the data they want without
swamping the spacecraft's telemetry system.

"Our approach emphasizes particular elements of the science
goals," Pendleton said. "At this meeting we will discuss
our status and why we favor this design."

Following today's review, the competitors will go home and
refine their concepts for the final competition. The draft copy
of the Announcement of Opportunity has been released for comments
by the scientists. The final AO will be issued in June, with
replies due in September. By early 2000, NASA plans to select
one concept for development and, ultimately, launch in 2005.